CFD's potential applications: a wind engineering perspective

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CFD's potential applications: a wind engineering perspective

Girma Bitsuamlak,

a

Emil Simiu

b

Civil and Environmental Eng. Dept and International Hurricane Research Center (IHRC),

Florida International University (FIU), Miami, FL

a

,

b

ABSTRACT:Boundary wind tunnels are industry-wide accepted, effective wind engineering tools. How-ever, slowly but surely, CFD is making important inroads. What can the wind engineer expect from CFD developments? One type of application concerns the characterization of the main features of meso-meteorological flows. In this area CFD calculations are likely to meet wind engineering needs. A second type of application where CFD can be at least as effective as wind tunnel simulations is the simulation of flows over terrain with topographical or orographic features, or with inhomogeneous surface roughness. This type of application is important for both structural engineering and wind power applications. A third type of application concerns the estimation of wind speeds in pedestrian areas, where CFD simulations have already been proven to be a good substitute for or complement to wind tunnel testing. Fourth, it is conceivable that CFD will be useful for the estimation of global aerodynamic loads acting on main wind force resisting systems, as opposed to local loads on cladding. Fifth, CFD has proven quite reliable in complementing experimental data on flow generation. The performance of passive devices has in many instances been predicted by CFD with a remarkable degree of accuracy.

1 INTRODUCTION

The evolution of computational wind engineering (CWE) based on computational fluid dynamics (CFD) is making numerical evaluation of wind effects on the built environment a potentially attractive proposi-tion. This is particularly true in light of the positive trends in hardware and software technology, as well as in numerical modeling. Significant progress has been made in the application of CWE to the evaluation of wind loads on buildings (e.g. Tominaga, et al. 2008a, and others). Working groups have been estab-lished to investigate the practical applicability of CWE and develop recommendations for its use for the wind resistant design of actual buildings and for assessing pedestrian level wind, within the framework of both the Architectural Institute of Japan (AIJ) (Tamura, et al. 2008; Tominaga, et al. 2008b) and European Cooperation in Science and Technology (COST) (Franke et al., 2007). Further, AIJ provides methods for predicting wind loading on buildings by the Reynolds Averaged Navier Stokes equations (RANS) and LES. Progress has also been made in the evaluation of wind load modifications due to topographic ele-ments (Bitsuamlak et al. 2004, 2006). Practical applications of CWE are widespread in areas such as pe-destrian level wind evaluation where only the mean wind speeds are required for the evaluation of pede-strian comfort (Stathopoulos and Hu, 2004). CWE applications on wind driven rain were reported by Blocken and Cameliet (2004). CFD wind flow studies of urban neighborhoods include Zhang et al. (2006) and others. While most studies mentioned above focus on straight winds, studies by Hangan and Kim (2008) focused on simulation of downburst. Other common uses of CWE include complementing of ex-perimental wind engineering research: Sengupta and Sarkar (2008) complemented their microburst and tornado wind simulator facility with numerical simulation; Moonen, et al. (2007) used CFD to assess the quality of wind tunnel flows and to design wind tunnels. Merrick and Bitsuamlak (2008) used numerical simulation to facilitate selection of an artificial surface roughness to be applied on curved surfaces of wind tunnel building models to compensate for Reynolds numbers deficits. Recently, researchers at FIU in collaboration with engineers at RWDI applied CFD to the design the phase III full-scale testing facility

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The Fifth International Symposium on Computational Wind Engineering (CWE2010) Chapel Hill, North Carolina, USA May 23-27, 2010

generically named “Wall of Wind” (WoW). Bitsuamlak et al. (2010) computationally assessed the effects of blockage pertaining to size of test buildings relative to the finite wind field size of WoW.

This paper discusses important inroads of CFD in wind engineering applications as complementary tools to boundary layer wind tunnel model scale testing and full/large-scale testing in facilities such as FIU’s Wall of Wind. What can the wind engineer expect from CFD developments? One type of applica-tion concerns the characterizaapplica-tion of the main features of meso-meteorological flows. In this area CFD calculations are likely to meet wind engineering needs. This type of application uses multi-scale simula-tion. The large temporal and spatial scale flows are simulated by using meso-meteorological models such as Weather Research Forecasting (WRF) models (Skamarock et al. 2005). The small-scale spatial and temporal characteristics near the ground surface and within the smallest grid size of the WRF model are simulated by high resolution CFD models based on RANS or LES. The CFD model can use WRF results as boundary conditions. A second type of application where CFD can be at least as effective as wind tun-nel simulations is the simulation of flows over terrain with topographical or orographic features, or with inhomogeneous surface roughness. This type of application is important for both structural engineering and wind power applications. A third type of application concerns the estimation of wind speeds in pede-strian areas, where CFD simulations have already been proven to be a good substitute for or complement to wind tunnel testing. Fourth, it is conceivable that CFD will be useful for the estimation of global aero-dynamic loads acting on main wind force resisting systems, as opposed to local loads on cladding. Fifth, CFD has proven quite reliable in complementing experimental data on flow generation.

2 CFD APPLICATIONS IN WIND ENGINEERING

2.1 Oncoming wind flows

Proper oncoming wind flow characteristics similar to those of natural wind conditions at the site of inter-est need to be used as an input to the simulation of wind flow around buildings, both in experiments and in computations. In current practice, codes and building standards such as ASCE 7-05 provide 3-s gust basic design wind speeds for open terrain conditions at 10 m elevation, derived largely from data at local airport stations. The wind speed at a particular site will then have to be derived from the basic wind speeds through proper exposure corrections that reflect the roughness of the ground surface. The ground surface roughness lengths are usually estimated by examining aerial or Google Earth (www.googleearth.com) photographs for each wind direction. These estimated values are then used in lo-garithmic wind velocity profiles at a particular site. For inhomogeneous upwind terrain conditions and for city centers this task is even more complicated. Some wind consulting offices use the ESDU (Engineering Science Data Unit) approach (ESDU 1993). ESDU uses equations fitted to data based on a numerical model developed by Deaves (1981). An equivalent surface roughness is obtained by considering fetch lengths and associated surface roughness lengths for each wind direction. The equivalent surface rough-ness is applied at the floor of a wind tunnel to generate wind test profiles. A better job can be done by per-forming a detailed numerical simulation that reflects the complexity of upwind roughness in urban areas and complex upwind terrain by using CFD simulations. Details of roughness lengths can be derived from GIS (Geographic Information Systems). For Florida, it is possible to use IHRC’s high resolution conti-nuous surface model produced with LIDAR, an airborne laser mapping system which employs laser-ranging technology and GPS positioning (Figure 1a). These topography and surface roughness data can be used to define the ground boundary of the large-size computational domain; the latter will subsequent-ly be used to assess the effect of surface roughness and upwind terrain on surface wind characteristics (Figure 1b). Details of this approach can be found in Abdi and Bitsuamlak (2010), a paper submitted to this conference. Thus, accurate topographical effects and ground surface roughness will constitute the ground boundary of the computational domain. Boundary conditions for this type of CFD simulations can be obtained from measurements at nearby weather stations. They can also be derived from meso-scale meteorological numerical models, e.g., WRF models, which usually have grid spacing on the order of 10 to 30 km that will be compatible with typical computational domain size for oncoming flow simulations.

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Figure 1 (a) IHRC LIDAR data (b) the effect of surface roughness on the oncoming wind speed profiles.

Figure 2. Numerical simulation of wind flow field over complex terrain.

2.2 Flow over topographic terrains

The common practice with regard to isolated hills with simple sinusoidal shapes, escarpments, or valleys is to use topographical factors from building codes and standards, usually referred as “speed-up” factors. For complex topographic situations, a separate topographical study at ~1:3000 scale is typically con-ducted in wind tunnels to assess the effect of the topography on wind speed and direction for each of a to-tal of 36 wind directions. The topographical factors are then applied as correction factors to wind tunnel test results conducted on buildings at, say, 1:400 scale. It is noteworthy that scale and hot-wire wind mea-surement issues in BLWT topographical studies at 1:3000 scale can produce errors in the areas of interest (near ground and circulation zones) comparable to or larger than errors associated with numerical mod-els. Previous experience (Bitsuamlak et al. 2004, and 2006) suggests that numerical studies will in the near future be able to replace topographical studies now conducted in wind tunnels. The sequential topo-graphical and building wind tunnel testing or numerical simulation has a fundamental shortcoming: in ac-tual conditions a building is affected by wind modified by the topography. Correcting the responses ob-tained for normal wind conditions (in the absence of topographic effects) by topographical correction factors could yield different results from those that would be obtained by testing the topography and the building at the same time in an integral fashion. This can be done efficiently in CFD approach. A numeri-cal simulation of wind flow over complex terrain is shown in Fig. 2. The maximum height of the terrain was 500 m and the surface roughness was zo = 0.7 m at full scale (Figure 2). A 167x150 mesh covering

3000 m x 8000 m was used. The gradual increases in velocity as the wind approaches the hill and the re-circulation region behind the hill were simulated by the CFD as shown in Figure 2b.

2.3 Urban wind flows, pedestrian level winds (PLW)

PLW can be described quite adequately in terms of mean velocities in the presence and absence of a new building within a specific urban environment. Although it can be argued that pedestrians are mostly af-fected by gust effects and mean wind speeds adequate indicators of discomfort, several major cities re-quire only that certain probabilistic criteria on mean (sustained) speeds be satisfied (Stathopoulos and

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Wu, 2004). Pedestrian level wind simulation is perhaps the most promising area for the use of CFD in wind engineering at present. This area is also an example of how CFD and wind tunnels can be used col-laboratively. An initial CFD study can be used to improve a design and indicate where possible problems may occur. Pressure taps can then be placed in the wind tunnel in the areas so identified.

2.4 Wind loads on buildings

Most numerical predictions of wind pressure loads on buildings have been performed for basic shapes in turbulent boundary layer flow representing low-rise and tall buildings, in part owing to the availability of experimental data (Lim et al., 2009). The focus of the numerical evaluation has been on both low-rise and tall buildings. Commonly studied full-scale low-rise buildings include the Silsoe Cube and the Texas Tech building (Senthooran et al. 2004). Studies carried out on high-rise building were performed by (No-zawa and Tamura, 2002; Tominaga et al. 2008a) and others. Huang et al (2007) studied the external aero-dynamics of the CAARC building and investigated flow patterns as well as mean and root-mean-square (rms) pressure coefficients on the building envelope. Most existing numerical studies for the assessment and evaluation of interference effects employed the commonly used Reynolds-averaged Navier-Stokes (RANS) equations. Huang et al (2007) and Dagnew et al. (2009) used Large Eddy Simulation (LES) and validated their numerical data with wind tunnel data.

However, major CFD simulation problems for wind load applications remain. Generally, numerical predictions in separation zones on buildings such as roofs, side walls, and leeward walls are considerably less accurate than those for windward faces (Stathopoulos 1997; Gomes et al. 2005). This is mainly due to the isotropic flow assumptions used in most common turbulence models, including the standard k-ε mod-el. In addition, studies have been carried out mostly for isolated buildings with a simple cubic shape, without considering the effect of the surrounding buildings and of topographic features. With other build-ings present in close proximity, the dynamics of the flow become much more complex and flow interfe-rence occurs (Khanduri et al. 1998). A holistic approach that accounts for upwind terrain effects and aerodynamic interactions with nearby buildings needs to be developed. Finally, there is a need to evaluate numerically internal pressures, which can significantly contribute to the total wind loads on roofs and ex-ternal walls, as well as on cladding and components. Inex-ternal pressures need to be investigated for differ-ent internal compartmdiffer-entalization, opening sizes and locations, and building shapes.

Wind induced responses are also affected by building shape and orientation and, for flexible build-ings, by their dynamic properties (mass, stiffness and damping). For most building projects the shape and orientation are driven by architectural considerations, functional requirements and site limitations, rather than by aerodynamics considerations. However, in some cases aerodynamics plays a significant role in determining the shape (Merrick and Bitsuamlak 2009). This can be in particular the case for high-rise buildings, where wind controls many aspects of the design. High-rise buildings can be susceptible to ex-cessive motion during wind events that can cause occupant discomfort and reduce the overall appeal of the structure (Kareem, 1999), as well as experiencing high loads, which can increase the cost of both the structural system and the foundation. There are various methods for the mitigation of wind-induced mo-tion and the reducmo-tion of wind loads on high-rise structures. Structural engineers generally opt for opti-mizing the structural system or increasing modal mass to reduce wind- induced motion (Kareem et al. 1999). Tamura and Miyagi (1999), Kawai (1998) and Dutton and Isyumov (1990) studied effects of cor-ner modifications on the aerodynamic forces of square cylinders. In many cases, studies provide reactive solutions aimed at remedying wind-induced problems observed when the structure is nearing design com-pletion. Published information on how to design structures for optimal aerodynamic performance is very limited. Computational simulations can play an important role in this regard. Computational approaches appear to be more promising for the estimation of wind loads on main wind force resisting systems, which are less sensitive to detailed local geometry and flow separation

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Figure 3 – Phase III WoW (a) computer model; (b) 1:15 scale WoW; (c) full-scale fans ready for construction; (d) CFD simulation; comparison of small scale and CFD for (e) wind velocity and (f) turbulence profile.

2.5 Complementing experimental testing

Other common uses of CWE include complementation of experimental research. Sengupta et al. (2008) used CFD to complement experimental Tornado Simulation research, Bitsuamlak et al. (2010) assessed blockage and proximity effects at the Wall of Wind facility, Merrick and Bitsuamlak (2008) used CFD to study the effect of artificial roughness on buildings with rounded shapes. Browen et al. (2006) used CFD to study the geometric scale effects for porous walls in wind tunnel testing. The following sections de-scribe recent CFD work carried by the authors’ research team.

2.5.1 Wall of Wind design

Central to FIU’s research is the development in stages of full-scale testing facilities of the type generically called Wall of Wind (WoW), capable of producing hurricane level winds, in conjunction with wind- dri-ven rain and wind-borne debris. As a first phase of this development effort, the International Hurricane Research Center (IHRC) team at Florida International University (FIU) has built a large-scale 2-fan WoW facility for testing small structures and assemblies, including roof fascias, barrel tile roofs, hurricane miti-gation products, and Florida Power & Light utilities. Building on this experience FIU has subsequently built a larger, more powerful Renaissance-Re 6-fan WoW generating large wind and wind-driven rain field. Further expansion and improvements on the current design of WoW using 12 electric fans are un-der way with support from the State of Florida Legislature. CFD simulations have been very useful in the design process. Different arrangements of fans (6 x 2, 4 x 3), for different flaring angles have been simu-lated. The arrangement that satisfied the target -- 140 mph sustained wind speed over a 20 ft by

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Figure 4 – Gable roof eve zone mitigation (a) tap lay out (b) mean pressure coefficient comparison before and after mitigation. Slope 2:12

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16 ft area -- was validated by a 1:15 scale WoW physical model (Fig. 3b). The comparisons between the CFD and the experimental profiles (velocity and turbulence intensity shown in Figs. 3e and 3f) show ex-cellent agreement, attesting to the importance of CFD simulations in developing testing facilities such as the WoW. A similar approach is used at the University of Western Ontario for the WindEEE (www.WindEEE.com) facility design. At the moment, work on developing passive and active flow man-agement devices is in progress using the small scale 12-fan WoW and CFD simulations.

2.5.2 Aerodynamic modifications

Aerodynamic modifications that use simple architectural details at corners to mitigate high suctions (neg-ative pressures) at roof and wall corner zones were studied at FIU using full-scale WoW tests. The per-formance of several proposed aerodynamic mitigation devices was assessed by comparing pressure mea-surements on the buildings at the locations of interest before and after installation of the aerodynamic mitigation retrofits. One such application was the use of Pergolas, which are common in Florida, to re-duce suctions at roof eaves. Prior to the full-scale testing a CFD simulation was carried out for several spacings of Pergola frames (Fig. 4) and details of the gutter. The best performing set was adopted for con-firmation test at full-scale. The CFD simulation significantly reduced the number of costly full-scale tests

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2.5.3 Blockage and proximity effect assessment

Numerical wind flow simulations have been carried out for flows around parallelepipeds of various sizes, located at various distances from the wind simulator, and immersed in numerical WoW and Atmospheric Boundary Layer (ABL) flow models. A blockage and wind simulator proximity effect study focused on the effect of the size of the test buildings with respect to the finite size of WoW wind field, and on the ef-fect of the building’s proximity to the wind simulator. The CFD simulations helped to prepare test guide-lines with regard to size and location of the test building in the WoW facility (Bitsuamlak et al., 2010).

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Figure 5. Individual roof paver; pavement installation; test specimen in front of WoW; CFD simulation

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2.5.4 Roof pavements

The main aim of the study was to help assess both external and internal pressure distributions through ex-perimental testing of roof pavements (Fig. 5) that rely on their dead weight to counterbalance the wind in-duced net uplift forces. A computer model was also developed and CFD simulations were carried out to highlight some of the flow mechanisms qualitatively and visually for easy interpretation by builders and product manufacturers.

3 CONCLUSIONS

CFD has proven useful, and has made inroads, for various wind engineering applications such as the es-timation of wind flow modifications due to topographic changes and urban roughness effects, non-synoptic wind simulation, estimation of wind pressure loads on structures, and pedestrian level wind si-mulations. Studies have also shown that CFD simulations can be used in conjunction with wind tunnel experiments and the design of wind engineering facilities. CFD simulation is also useful for visualizing complex wind /structure interactions.

4 ACKNOWLEDGEMENTS

The first author would like to acknowledge financial support by the State of Florida Legislature, the Flor-ida Center of Excellence grant, and the National Science Foundation through a CAREER award under Grant Number 0846811 to the first author. The support by Renaissance Re of the Wall of Wind Project is also gratefully acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation nor Renaissance Re.

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CFD's potential applications: a wind engineering perspective